Implement bytecode patching to solve NoSuchFieldError issues

[aasitnikov]

Because we embed modules into one fat-aar, aapt wont generate R class for these modules. So, fat-aar generates R.java classes by itself, and puts it into "lib/r-classes.jar" inside resulting aar.

Imagine we have a resource, that was present in one version of some other library, and was removed in next version. For example, take TextAppearance.MaterialComponents.Tab - it was a private resource of MaterialComponents:1.0.0, and it was removed in MaterialComponents:1.1.0. If we compile our using version 1.0.0, fat-aar generates following R.java class:

package my.emdedded.library;

public final class R {
    public R() {
    }

    public static final class style {
        ...
        public static final int TextAppearance_MaterialComponents_Tab;
        ...

        static {
            ...
            TextAppearance_MaterialComponents_Tab = my.fat.library.R.style.TextAppearance_MaterialComponents_Tab;
            ...
        }
    }
    ...
}

When consumer of this aar uses MaterialComponents 1.0.0 and compiles an apk, aapt will generate my.fat.library.R.java class with TextAppearance_MaterialComponents_Tab field in it. But if consumer compiles it's apk with MaterialComponents:1.1.0, my.fat.library.R.style.TextAppearance_MaterialComponents_Tab won't be generated by aapt, when compiling app module. So, in runtime, when my.emdedded.library.R class first loaded, NoSuchFieldError will be thrown in static initialiser.

Fat-aar plugin can solve this problem using bytecode patching. Plugin should patch classes from embedded aar, changing imports of R classes to match fat-aar's package. In example above, in every class of embedded library, references to my.emdedded.library.R should be replaced with my.fat.library.R. Then there will be no need to generate R.java classes into r-classes.jar.

If implemented, this change will solve a lot of current issues: #75, #90, #136, #161, #170 and probably #169, #180

[aasitnikov]

I've created Transformer, that implements bytecode transformation described above. It uses Transformer API from AGP, and uses JavaAssist to patch bytecode.

To apply it, write the following lines in build.gradle:

android.registerTransform(new EmbedRClassesBytecodeTransformer(
        ["my.embedded.library"], // list all embedded modules here
        "my.fat.library"
))

This transformer can be used in the plugin as is - plugin knows packages of current and embedded modules, and can register transform easily.

UPD: In newer versions (1.2.20+) we need to specify order between tasks

// Explicitly specify order between bytecode transformer and fat-aars mergeClasses
// If transform would occur before mergeClasses, output won't have emdedded classes
android.libraryVariants.all {
    def name = it.name.capitalize()
    tasks.named("transformClassesWithFatAarRTransformFor$name").configure {
        it.dependsOn(tasks.named("mergeClasses$name"))
    }
}

[yallam08]

@aasitnikov do you have a working demo where you implemented this?

[aasitnikov]

@yallam08 Made a little example app, where this transformer is used https://github.com/aasitnikov/fat-aar-bytecode-patching

[aasitnikov]

@kezong What do you think of adding this bytecode transformer to plugin?

[kezong]

@aasitnikov This is a good idea. You can pull request for the bytecode transformer. To be safe, best to add a switch that can back to the original method.

[aprz512]

@aasitnikov the demo works fine, even if I didn't uncomment the register transform statement.

[kezong]

It is supported in 1.3.1